The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with an electrically conductive copper oxide-containing material, preferably an electrically super conductive copper oxide-containing material. More particularly, this invention relates to a new process for producing thin and thick superconductor films.
A number of techniques may be employed to provide conductive copper oxide coatings on inorganic substrates. For example, a high temperature sintering process may be employed. This process comprises contacting a substrate with an oxide source comprising for example a copper-oxide component, a yttrium oxide and barium oxide source material and contacting the components with an oxygen-containing vaporous medium at sintering temperature conditions effective to form the conductive copper oxide coating on the substrate.
However, superconductor materials are very difficult to work with, especially because of their brittleness. Once the material has undergone the sintering process, it is very difficult to from the material, particularly since the material is usually a ceramic typical of most superconductors.
Such superconducting materials in their finished states are extremely brittle, unmachineable and unbendable. Whatever form they are in after sintering is the form they stay in and little or no alterations have been developed.
Conventional superconducting materials, such as niobium-titanium and niobium-tin operate at liquid-helium temperatures (4.5 Kelvin) for cooling. New superconductors, currently under development, operate in liquid nitrogen, i.e. an expensive cryogenic fluid, at temperatures of 77 Kelvin or higher.
Conventional processing of copper oxide conductors, particularly for superconductors include:
(1) Substrate depositions, where ion beams are used on zirconium and sapphire substrates in various types of atmospheres. This method is presently being developed for microprocessor applicable films. A film is placed on to the flat surface, (not on a three dimensional surface) of a microprocessor ship; (2) Fiber (whisker) growth methods, which produce pure superconductor fibers using a laser heated growth method; and (3) extrusion.
A limitation of substrate deposition is the high cost of processing .and expensive equipment required, i.e., sapphire substrates, ion beam deposition furnaces, lasers. However, the prior superconductor processes are still in an early stage of development due to the recent discoveries in copper oxide based superconductor.
One process undergoing development for applying a superconductor layer or material onto a copper wire, includes surrounding a copper wire with a yttrium-oxide and barium-carbonate powder pack. The powder is fired similar to other conventional methods of processing of bulk superconducting material.
During the process, the outer layer of the copper wire is oxidized producing a copper oxide layer around the wire. The yttrium and barium components react with this copper oxide by diffusion to produce a superconducting compound, a layer or an outer coating.
The results published to date showed a 5- to 10-micron layer (depending on firing time) of material in which all three of the constituent elements were present, as observed on the copper wire by a scanning electron microscope. Whether or not they were present consistently and continuously in the appropriate crystal from was not determined, but the Energy Dispersive Analysis indicated a correct element ratios.
It was also observed that the material could possibly be in patches or the crystals slightly removed from each other, thus disabling a continuous circuit. SEM analysis revealed the porous nature of the ceramic material and the agglomerated, grainy mix of the various phases within the material.
The conventional ceramic processing techniques have been adopted to prepare kilogram size powder batches and to fabricate bulk superconductors. In most cases, yttrium oxide, the oxide, peroxide, hydroxide or carbonate of barium and the oxide or carbonate of copper are used as precursors for the YBa.sub.2 Cu.sub.3 O.sub.x compound. Appropriate quantities of these precursor powders are mixed effectively by ball milling. Carbonates and oxides of yttrium, barium and copper have little solubility in water and are readily mixed in an aqueous vehicle.
Calcined powders can be formed into different shapes and configurations by various forming techniques including dry pressing, tape casting, screen printing and extrusion. The dry pressing method has been used to fabricate bulk superconducting parts with dimensions ranging from 90.1 to 20 c.m. Superconducting wires have been prepared by extrusion. Superconducting ceramic tapes (-20 to 100 um thick) have been prepared by a tape casting technology similar to that used in fabrication of multilayer ceramic capacitors and ceramic packages for integrated circuits. Layers of superconducting and insulating tapes can be laminated to form multilayer device structure. Superconducting lines and pads have been prepared by the screen printing process. A viscous paste is first formed by mixing a superconducting powder with organic binders. The paste is then printed through a patterned fine-meshed screen onto a substrate to form thick film superconducting patters having -5 to 20 um thickness.
The formed superconductor parts and circuit patterns are then fired at 900.degree.-1000.degree. C. to densify the ceramic. Later, proper oxidation anneal is usually necessary to provide a sufficient oxygen content for the required superconducting device properties.
Some factors are known to contribute to a better superconducting material and these factors include a higher density resulting in improved mechanical properties and a highly oriented texture in this films exhibiting a high critical current density.
Wires and cables of the ceramic materials are usually made from molded, extruded, or compressed polycrystalline powders. The flow of current between the polycrystalline grains is limited by boundaries between grains that act as "weak links" and the grains' directional anisotropy, or nonuniformity, with respect to current flow in the crystal. Current flow is impeded when it must follow a wandering path through randomly oriented grains. Aligning the grains can help to increase the current-carrying capacity of the ceramic material.
A significant problem with currently available thick oxide materials is their behavior in applied magnetic fields. Superconductors are either Type I or Type II materials . Both types exclude magnetic fields and are superconducting until a critical field level is reached. Above this level, Type I materials become nonsuperconducting. Type II materials, however, continue too superconduct, but allow magnetic flux to penetrate portions of the crystal lattice. Only when an upper critical field is reached do the Type II materials become nonsuperconducting. Most high-temperature superconductors are Type II materials.
Although the new superconductors have extremely high upper critical field limits, the "flux lattice," which is how the magnetic fields penetrate the superconductor, is unstable. Unless the flux lattice is "pinned," magnetic forces from circulating currents act on the magnetic field lines and cause the flux lattice to move. This movement, or flux creep, creates resistance to current flow.
It is generally believed that, because thin films of the materials can carry large currents, flux creep is not an intrinsic property of oxide superconductors material. There is a need to be able to manufacture film, particularly thin films and to be able to control boundaries between grains. There is a need to be able to manufacture film, particularly thin films and to be able to control boundaries between grains.
The above conventional sintering processes are examples of processes in which the oxides are generally formed first, particularly as powders, followed by sintering on flat and or smooth powder accessible surfaces.
There are significant limitations inherent in the prior art processes. For example the processes are generally based upon conventional ceramic processes and the use of oxide precursor powders and forming. Powders are consolidated themselves or deposited on a substrate followed by compaction and sintering. These limitations are particularly apparent for the processing of non-flat surfaces and where coating uniformly and reduced grain boundary deleterious effects are essential. For example, particularly with non-flat surfaces, portions of a substrate, particularly internal surfaces, which are shielded from the copper oxide powder e.g., such as pores which extend inwardly from the external surface and substrate layers which are internal at least partially shielded from the depositing copper oxide source by one or more other layers or surfaces closer to the external substrate surface being coated, or because of such external surfaces closer proximity to a source system do not get uniformly coated, if at all, in solid/solid type of sintering processes. Such shielded substrate portions either are not being contacted by the powder source during processing or are being contacted, if at all, not uniformly by the powder source during processing and/or the processing time required is excessive or the process not applicable to continuous productions.
Although the sintering process is useful for coating a single flat surface, for the reasons noted above this process tends to produce non-uniform and/or discontinuous coatings on three dimensional surfaces having inner shielded surface and/or the processing is difficult or time consuming. Such non uniformities and/or processing drawbacks are detrimental to the electrical and chemical properties of the coated substrate.
A new process, e.g., a "non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with conductive copper oxide which surfaces would not be directly exposed to copper oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step and/or which improves the overall processability to conductive component and article and/or type of substrate to be coated. In other words, a "three dimensional" process coats coatable substrate surfaces which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate during processing, e.g., the surfaces of the internal fibers of a porous mat of ceramic fibers and/or improve the overall processability from a time and/or type of substrate processing standpoint.